Remove path_hash everywhere

* Add a c14n_path to host_paths to compensate
* Use canonicalised path instead
> I think it's obvious that it is our canonical. Calling it c14n serves to obfuscate rather than identify.

It's worse even than obfuscation - `c14n` is an abbreviation for "canonicalization", not "canonical". Let's change it.

> I'm not sure why canonical in a filesystem context would come into it as the app (and this model) is all about URLs.

Because `path` is in the column name - this isn't a URL, despite the fact it's part of one, and "canonical" in a URL path sense can mean the same as the filesystem sense (i.e. `/something/down/here/../../thing.html` is equivalent to `/something/thing.html` whether it's in a URL or a filesystem), and I didn't want to give the impression our c14n did that, because it doesn't. Unless it does, of course; it's been some time since I did anything with it.